The FCC has begun an investigation into allegations of pay for play at Clear Channel Communications, CBS Radio, Entercom Communications, and Citadel Broadcasting, after it was unable to reach agreements with the four over fines for their previous practices. The agency has now sent out letters of inquiry, requests for documents that begin the process of formal investigation. The federal process follows on the heels of New York Attorney General Eliot Spitzer's lawsuit against Entercom. Read more here and here.